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CBF 125 Oil Consumption

Blog Date 3 October 2017

The old girl is starting to show her age. At 67,000 miles the CBF125 is looking very tired but then it always has, now her internals must be starting to look tired too.

The corroded aluminium and rusty exhaust of Ren's CBF125
It's no longer a spring chicken these days. 

For all this time I've changed the oil every 2,000 miles. As the miles have rolled by I've slowly become aware that I need to top up the oil every now and then between services. More recently I've noticed I'm having to do this more often and I'm putting in larger quantities of oil.

After the last oil change at 66,000 miles I made a mark on the oil tub, noting the level. Since then I've topped up twice and after 1,300 miles I must have used around 1/2 a litre of oil. These are not accurate measurements but as a rough guide I now consider the oil consumption to be "notable".

The marked level and the current level visible on the side of the oil container
I'm reckoning on half a litre difference give or take.

Bearing in mind that each standard oil change requires *just* over a litre AND the specified service interval is 2,500 miles, it would appear I'm using roughly a litre between recommended changes. As such there seems to be little point in changing the oil, it's running more like a total loss 2-stroke these days! So should I just ignore future oil changes, safe in the knowledge that the oil is always fresh because it's always burning it?

Yes - and no. There's more than one reason to change engine oil. The primary reason to change oil is because the oil degrades but with the regular addition of fresh oil this degradation will be kept in check. However oil also acts as a medium into which dirt and worn metal is suspended. If these are not burnt off then as I continually add fresh oil the concentration of suspended gunk will increase with time.

As such I WILL keep on changing the oil but I'm considering whether or not to move to changing the oil once every 3, 4 or even 5 thousand miles. I'm trying to strike a balance of not just wasting oil by refreshing already refreshed oil against the potential build up of unwanted particles. 

What? Did someone suggest I dismantle the engine and recondition the piston, rings and barrel? How very dare you! That would cost money and I'm not entirely convinced what's left of the CBF125 is really worth it.

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Reader's Comments

Daf said :-
Though the diminutive CBF is getting on a bit, you've had a bloody good run from it! when the oil consumption gets too ridiculous (or you lose compression and it doesn't run/smoothly) it would be really interesting to see you overhaul the engine/see if it's possible for a home mechanic to do so! Would make a really interesting article series!
04/10/2017 10:37:14 UTC
CrazyFrog said :-
Run it till it drops Ren, it certainly doesn't owe you anything does it?
04/10/2017 12:48:00 UTC
Ren - The Ed said :-
Nah the owd beast has been and still is a good 'un. It owes me nowt and I probably owe it a lot.

Daf - there will be an autopsy. I myself and perhaps quite a few other people would like to see what the internals are like. BUT...but if it's all the same with you I will try and procrastinate that autopsy for as long as possible. I'd like to reach 100,000 miles but I've still got 32,500 miles to go before I get there. Actually I'd like to see 200,000 miles! Actually let's go into lorry distances here, I'd like to see half a million miles.

I don't want to end up with a "Trigger's Brush" situation though. If the motor lets go then that will be that. I'm not thinking of replacing the motor then replacing the fuel pump then replacing the computer then replacing the injector just so I can point at the bike and say "It's done XXX thousand miles that". There are other 125s out there I'd like to destroy - I mean try out.
04/10/2017 12:59:38 UTC
Bob said :-
Is it down on power and / or hard to start?
It might just be stem seals, rather than rings / bore.

My XR125 (23K miles) uses a little oil but it's only the stem seals, it smokes a little on start up but doesn't smoke under normal use and compression is still OK.

That might be the place to start - a compression check.
The other thing would be to see if you're getting oil in the airbox which would indicate blow-by from worn rings.
05/10/2017 10:34:56 UTC
Ren - The Ed said :-
I...I suspect it is down on power although it's not hard to start. If it is down on power it happens over such a long period of time that you don't notice, much like watching your children grow. If I were to ride a much newer CBF125 I may be surprised, much like when a relative who's not seen the kids in a while exclaims "Oh my haven't they grown!"

I'm not seeing any smoke at all, either on startup or general riding. When Sharon was following me the other day she never mentioned a cloud of black soot emanating from my derrière.

The next time I change the spark plug I may do a compression test, I have one of then lying around. I'll also stick my nose into the airbox, I'd not considered looking for oil in there.

Thanks Bob, good calls.
05/10/2017 11:15:28 UTC
ybr125 said :-
Move to a cheap 15w40 oil, it should cut down on the consumpton problem.
05/10/2017 11:51:16 UTC
Ren - The Ed said :-
I'm already running Halford's basic 10w40 motor oil. Are you suggesting the consumption won't be a problem because the oil is cheap or that the thicker cold startup viscosity will reduce consumption?
05/10/2017 12:48:26 UTC
ybr125 said :-
The 15w40 oils are genneraly stronger, thicker, resist shearing better. I would especially recommebd diesel engine oils like Movil Delvac mx 15w40 in your bike. thicker oil should resist consumption much better. 20w50 should also be fine for a tired engine and will give better oil pressure due to larger clearances which developed.
05/10/2017 22:05:24 UTC
Ren - The Ed said :-
I don't know if you are aware but the CBF125's crank is a pressed up needle roller bearing affair. While these are very strong oil choice is critical.

As the needle roller rolls around it forms a ridge of oil in front of each needle. If the oil is too thick the oil ridge won't move out the way fast enough. This apparently causes high pressures and damage. I found this out on a CD200 Benly with noisy tappets. To quieten the tappets as a youth I'd used an oil thickener.

I reckon a 20w50 would be ok but I'm loathe to try at risk of damaging the crank. A 15w40 would be ok as the thickness is the same at running temperature.
06/10/2017 05:57:52 UTC
Jaska said :-
Problem is probably valve stem seal in exhaust valve. That is most likely to cause problems in small high revving engines. Like honda monkey mopeds had same issues a lot.

If seal is not changed in time it might cause the valve to burn which will leave you by the road. And will be more bit more expensive to repair than just changing stem seals.
15/02/2018 18:23:49 UTC
Ren - The Ed said :-
Cheers Jaska. You've got me paranoid now. I was told a long time ago that valve stem seals would usually cause smoke on startup that would clear after being run for a few minutes. That is not the case with my bike. However I am fully open to the notion that my information could be wrong.

I'll look further into this. I can't quite see as yet how a little oil dripping down the valve stem would cause the valve to burn out. Again I need to do my research.

Luckily it is not *too* difficult to change the stem seals on the CBF125 but I am loathe to disturb the motor unless I have good cause too.
16/02/2018 10:04:11 UTC
Ian Soady said :-
In my experience, failing valve seals (particularly inlets) show up by smoking on the overrun as that's when you have a high vacuum in the inlet tract and oil gets sucked in by it.

16/02/2018 10:07:37 UTC

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